A transmission of this kind is known from EP-A2-280 481 and has two change speed stages which are connected in series between the driving element and a driven element. The driving element is formed in this known transmission by the pedal crank of a bicycle and the driven element is formed by toothed ring associated with this pedal crank. The stepless speed change in EP-A2-280 481 comprises a cam assembly whose eccentricity is adjustable relative to a center shaft. The device includes a carrier rotatably arranged about the eccentric cam assembly, rotary members on input and output sides, and friction pieces arranged in two rows, each row including a plurality of the friction pieces. The friction pieces in one row are rockably connected to the rotary member on the input side and the free ends of the friction pieces engage the inside of the carrier. The friction pieces in the other row are rockably connected to the carrier and the free ends of these friction pieces engage the inside of the rotary member on the output side.
In all known steplessly variable transmissions of this kind, and indeed also with single stage transmissions, the problem exists that speed fluctuations arise as a result of the sequential force transmitting engagement, of the coupling elements which take place in each case, and these can cause torsional oscillations and undesired loadings, and also disturbing noises. When using such steplessly variable transmissions in bicycles these fluctuations can also be perceived during pedalling as disturbing jolt-like sequential fluctuations of movement.
These fluctuations which occur with such transmissions depend on the layout or geometry of the particular transmission, and in particular also depend on the number of the transmission elements that are used in the form of pivotal levers provided with coupling elements. The greater the number of such pivotal levers can be selected the smaller are the said fluctuations, however an increase in the number of the pivotal levers and coupling elements not only leads to a substantial increase in cost, but above all also to a more complicated overall construction, and indeed also to an increased space requirement which is not acceptable for many applications.
Furthermore, a speed changing range which is as large as possible is required of such transmissions and design limits exist when using a single stage transmission. In accordance with EP-A2-280 481 two change speed stages are connected in series in order to enlarge the change speed range. In this manner one does indeed succeed in obtaining a larger speed changing range, since the total transmission ratio which can be achieved with a series connection of this kind corresponds approximately to the product of the speed changing ratios of the two individual stages, however the substantial disadvantage exists that, with a series connection of this kind, very large aperiodic variations or transmission fluctuations occur in the transmission part between the input and output members.